DISTRIBUTION OF ANIMALS 529 



barrier to forms adapted to life in the open. (2) Climatic conditions 

 often form barriers. Though temperature, generally speaking, is not 

 a serious barrier to many animals it may be an indirect factor in their 

 distribution by Hmiting the growth of plants upon which they feed and 

 by shortening their active season to such a degree that they cannot pass 

 through their entire life history and thus reproduction cannot take place. 

 Low humidity may be a barrier limiting the distribution of animals which 

 cannot withstand the drying to which they are subjected. (3) Lack of 

 proper food often forms an insuperable barrier for animals which feed 

 upon certain types of vegetation and the area of distribution of which is 

 strictly limited by the distribution of such plants. (4) Lack of locomotor 

 ability is also a barrier to rapid dispersal, although over a long period of 

 time it may not prevent the spread of animals into all areas suitable for 

 their existence. 



574. Modification of Types. — As animals have spread from their 

 point of origin and have met varying environmental conditions they have 

 become modified and adjusted to the conditions, and thus what was origi- 

 nally one form has been developed into a number of forms. These modi- 

 fications may indeed be such as to permit their possessors to surmount 

 barriers which otherwise would have prevented dispersal. 



575. Periodic Migration. — Periodic migration may be defined as the 

 repeated movement of animals from one place to another, at more or 

 less regular intervals, participated in by all of a species or by all of those 

 occupying a certain area. This excludes the movement of large numbers 

 when it is not repeated, the gradual dispersal of species, and the chance 

 wandering of individuals. In this restricted sense it is found throughout 

 the animal kingdom, being confined to those organisms capable of loco- 

 motion and being exhibited by them in proportion to their power of 

 movement. Migration is often associated with the search for food or for 

 proper conditions for rearing young. In many animals it is the expres- 

 sion of an instinct, involving factors which initiate the movement and 

 others which direct it. Among the initiatory factors are hunger, tem- 

 perature, and light conditions; the functional activity of the reproductive 

 organs; and a desire for home surroundings. Among the directive fac- 

 tors are relative temperatures, wind direction, water currents, and a sense 

 of location. Reference has been made to the limited vertical migration 

 of certain crustaceans (Sec. 303), which is shared by a large number of 

 aquatic forms belonging to several phyla. The migration of birds has 

 also been discussed (Sec. 434). 



Among the invertebrates the most extensive and best known migra- 

 tions are those of locusts, which have taken place in past times in Asia 

 Minor, southern Africa, Argentina, and the western United States. 

 These are due to lack of food in high barren regions and are directed 

 toward lower and more fertile areas. They occur at irregular intervals. 



